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The CHP inspects school bus fleets yearly. The CHP inspected 29 of Modesto City Schools' 60 buses in mid-January.

The four grounded buses violated the safety inspection with brakes out of adjustment; a loose mounting bolt for an axle's longitudinal torque rod; an underinflated inside tire; and air loss in a brake chamber, the district said.

Officials did not have details Thursday of the minor violations but said they can include such things as graffiti, trash on the floor or a window that does not work.

Miguel Galvez, the school district's director of facilities development and support, said eight to 10 of the 29 buses had violations. He could not give a precise number Thursday.

The CHP confirmed it conducted the annual safety inspection but declined to comment in detail until the inspection is finished March 22.

Galvez said the number of buses with violations during the most recent inspection "was a little higher than normal" compared with prior years.

He attributed the higher number this year to an aging fleet. The typical Modesto City Schools bus is about 20 years old, he said, adding that school districts across the state face the same problem.

He said Modesto's older buses take a beating on local roads that are in less than ideal condition.

"The items where we were cited are related to the conditions of the roads," Galvez said.

District officials said the district has not cut its transportation budget in recent years, despite having to cut millions from its budget because of declining enrollment and state cuts to education.

The district has allocated $900,000 a year for the next six years to replace aging buses, said Emily Lawrence, the district's spokeswoman.